Can't speak to the comforter and pillows but most places in Massachusetts will not/cannot take used mattresses as donations. If you run a search here, it looks like the option closest to the city is out in Walpole.

That said, Cambridge will pick them up for free as part of their mattress recycling program. You can find out more here.

Whatever you do, just don't be one of those derps who dumps their used mattress in front of a Salvation Army in the middle of the night on their way out of town.

In addition to donating, there's the option of just putting them up on Facebook marketplace, craigslist, or one of the buy nothing Facebook groups.

The city will pick these items up for recycling. Schedule a mattress pickup here -

https://www.cambridgema.gov/Services/mattressrecycling

and clothing/textile pickup here -

https://www.cambridgema.gov/Services/textilesrecycling

There is also a collection bin for clothing and textile recycling at the corner of Broadway and Fayette St. (possibly other locations too but this is the only one I know of).

Cambridge has multiple Buy Nothing groups on Facebook. I highly recommend posting there!

Someone living in a tent could probably use them. I always feel weird about approaching homeless camps to offer things but I usually find very grateful recipients.

Perhaps https://www.furnishinghopema.org/furniture ?

I have not had any luck getting anyone to accept donated pillows around here, which I get.

If I lived closer I’d take them, for when company comes. I just moved in and am low on textiles due to having mold in a previous dwelling. I’m at aquarium.